This invention relates to a process for the aromatization of chlorotetrahydrophthalic anhydrides to form chlorophthalic anhydrides. Substituted phthalic anhydrides are valuable raw materials for the synthesis of useful products. These anhydrides are utilized as intermediates in the synthesis of organic polymers, particularly polyimides, dyes, plasticizers, and in other uses.
Procedures are known for preparing substituted phthalic anhydrides. U.S. Pat. No. 4,560,772 discloses the reaction of 4-methyltetrahydrophthalic anhydride with excess sulfur and a catalytic amount of zinc oxide and 2-mercaptobenzothiazole to produce 4-methylphthalic anhydride and hydrogen sulfide.
U.S. Pat. No. 4,560,773 discloses a similar reaction between the electron-rich 4-methyl-tetrahydrophthalic anhydride and bromine in the presence of a catalytic amount of an acid acceptor such as dimethylformamide or pyridine in the liquid phase.
U.S. Pat. No. 4,709,056 discloses the dehydrohalogenation of 4-halo-4-flourohexahydrophthalic anhydrides through the use of a basic alumina catalyst in a liquid phase to produce two isomers of 4-fluorotetrahydrophthalic anhydride. The patent further discloses that the mixture of the isomers or the separate isomers may be dehydrogenated to fluorophthalic anhydride using a palladium on a carbon catalyst.
Ohkatou et al, Sekiyu Gakkasishi 1979, 22(3), (J. Japan Petrol. Inst.,) 164-9 (1979) (CA 91:74241e) discloses the dehydrogenation of hydrocarbons using an activated carbon bed to produce the corresponding olefins. The mechanism of the reaction using cyclohexane and cyclohexene were studied using a pressure flow technique.
Bergmann, J. Amer. Chem. Soc., 64, 176 (1942) discloses the aromatization of tetrahydrophthalic anhydride products of Diels-Alder reactions. The author discloses that dehydrogenation occurs when the tetrahydrophthalic anhydride product is boiled in nitrobenzene. However, it is further disclosed that dehydrogenation does not occur when p-bromonitrobenzene, p-chloronitrobenzene, or m-dinitrobenzene in xylene is employed. Moreover, it has been shown in our laboratories that when the halotetrahydrophthalic anhydrides of this publication are dehydrogenated in nitrobenzene, a portion of the nitrobenzene is reduced to aniline. The aniline reacts with the anhydride group of either the starting material or product to form imides and thus lower the yield of desired product.
Skvarchenko et al, Obschei Khimii, Vol. 30, No. 11, pp 3535-3541 (1960) disclose the aromatization of chloro-substituted tetrahydrophthalic anhydride by heating with phosphorus pentoxide. In the aromatization process described however, decarboxylation also occurs with the formation of the corresponding chloro-substituted benzene compound. The preparation of tetrahydrophthalic acids and anhydrides and various methods for dehydrogenation and aromatization thereof are reviewed by Skvarchenko in Russian Chemical Reviews, November 1963, pp 571-589.
P. P. Fu and R. G. Harvey disclose in Chem. Rev. 1978, 78 (4), 317 that platinum and palladium either as finely divided free metals or supported on activated carbon, are the most generally satisfactory catalysts for the dehydrogenation of hydroaromatic compounds. Attempts were made in our laboratories to use palladium on carbon as a catalyst for the aromatization of the halotetrahydrophthalic anhydride of this invention. The expected aromatization did occur; however, the catalyst also produced hydrodechlorination and the product produced was contaminated with significant phthalic anhydride or monochlorophthalic anhydride. Similar results were obtained with platinum or rhodium catalysts.
The published European patent application EP 89 10 3248.84, (European Patent 330,219) discloses a process for the aromatization of 4-chloro-tetrahydrophthalic anhydride in the presence of air and activated carbon to form 4-chlorophthalic anhydride. The process may be conducted in the vapor phase in the range of 200.degree. to 400.degree. C., or in solution from 200.degree. to 300.degree. C. Having the proper amount of air in the reaction is important because the oxygen in the air unites with the hydrogen atoms removed in the aromatization reaction to form water. When the reaction is run in solution, the water produced must be removed since it is known that water hydrolyzes phthalic anhydrides leading to reduced yields.